1. Field of the Invention
This invention relates to flare gas stacks for waste combustible gases from industrial processes including oil refineries, and more particularly to the control of purge gas thereto.
2. Brief Description of the Prior Art
It has heretofore been proposed to supply purge gas to a flare stack to prevent the occurrence of explosions in the stack.
Reed, in U.S. Pat. No. 3,741,713, shows a purge gas admission control for a flare system as a function of the temperature, a temperature sensing element being mounted in the stack or flare stack or in the supply duct which is connected to the stack or flare stack.
While reference is made to the use of molecular seals in an effort to prevent downflow of air into the stack none of the important variables involved are recognized or utilized to control the flow of the purge gas into the stack.
Reed et al., in U.S. Pat. No. 3,901,643, show a temperature-pressure activated purge gas flow control system for flares which adds to the temperature control system of U.S. Pat. No. 3,741,713, a control based on the pressure of the gas being supplied to the stack. This system has a pilot fixture providing a constant flame but with no provisions for controlling the pilot and no signals to indicate whether the pressure controlled valves or the temperature controlled valves are functioning. In FIG. 1, a water seal is shown at the base of the stack which is intended to prevent backflow from the stack into the supply pipe but this would not be effective to prevent entry of air into the stack itself, visual check of the liquid level in the water seal may be made or a liquid level operated alarm may be used.
Weinman et al., in U.S. Pat. No. 3,924,605 employ wind sensors to effect closing of the top of the stack.
The purge gas control systems heretofore proposed leave much to be desired in that they fail to take into account conditions at start up, variations in windspeed at or near the top of the stack, failure of steady state or transient purge flow, oxygen content of the gases advancing in the stack, monitoring of the purge gas and waste gas flow, and control of the pilot and its ignition to avoid pilot operation which could cause an explosion in the stack. The prior systems also did not give any indication of the prevailing conditions or their operating status.
The control system of the present invention overcomes the shortcomings of the purge gas control systems heretofore available with increased safety for operating personnel, with decreased cost for personnel and with resultant energy conservation and savings due to reduced utilization of high cost purge gas.